1. Field of the Invention
The present invention relates to a solid-state image pickup device and a fabrication method thereof, and, more particularly, to a solid-state image pickup device having an antireflection film formed on a surface of a microlens, and a fabrication method for fabricating the solid-state image pickup device.
2. Description of the Background Art
Recently, a solid-state image pickup device, which has advantages such as compactness, reduced weight, long-life, reduced persistence of vision, and reduced power consumption, has been rapidly widespread as an image pickup device incorporated in a digital video camera, and a digital still camera.
Some of the solid-state image pickup devices include a microlens made of a transparent material so as to enhance light-collection efficiency. A microlens is typically made of transparent resin such as acrylic resin or polystyrene, and a reflectance of the surface of the transparent resin is about 10%. In order to increasingly enhance the light-collection efficiency, increased reduction of the reflectance has been conventionally sought.
For example, Japanese Patent No. 2719238 discloses a method for forming an antireflection film on a microlens in the Langmuir-Blodgett method (hereinafter, referred to as “LB method”), or the water surface casting method, so as to reduce a reflectance of a surface of a microlens. Hereinafter, a coating method for the antireflection film disclosed in Japanese Patent No. 2719238 will be briefly described with reference to drawings.
FIGS. 16 to 18 are diagrams illustrating process steps of a conventional LB method disclosed in Japanese Patent No. 2719238.
The LB method is applied when a highly-volatile solvent having a low molecular weight is used. The solvent is prepared by substituting fluorine atom for hydrogen atom, in ether or ketone. In this solvent, a fluorine-containing resin (for example, a fluorine-containing epoxy resin, polyester resin, methacrylate resin, or phenolic resin) is dissolved such that the concentration of the obtained solution is smaller than or equal to 10% (preferably, smaller than or equal to 5%).
When the fluororesin solution obtained through the preparation described above is dropped onto water surface, a great part of the highly volatile solvent contained in the solution evaporates, and the fluorine-containing resin remains on the water surface as a monomolecular film as shown in FIG. 16. Surface pressure is sufficiently applied, to the monomolecular film remaining on the water surface, by using a floating element and a weight so as to obtain a solid film.
Further, as shown in FIG. 17, a substrate, such as a wafer or a chip, is moved up and down through the water surface so as to transfer the monomolecular film onto the substrate. As a result, a surface of a microlens on the substrate can be coated with a fluorine-containing resin film (antireflection film). Further, as shown in FIG. 18, the antireflection film having a desired film thickness can be formed by repeating the up and down movement of the substrate.
Further, a process for removing excess solvent by, for example, applying heat may be performed, as necessary, so as to enhance adherence between the substrate and the fluorine-containing resin film. In the LB method, it is important to appropriately select a type of the fluorine-containing resin and set the concentration so as to obtain a uniform film thickness.
On the other hand, in the water surface casting method, a thin film having the thickness of several tens of nanometers is formed by applying a few drops of fluororesin solution on water surface. Thereafter, as in the LB method, a substrate is moved up and down through the water surface so as to coat a microlens with the thin film (antireflection film). Alternatively, the thin film (antireflection film) may be transferred onto the substrate by moving the horizontally held substrate so as to be as close as possible to the thin film on the water surface, and then slightly tilting the substrate such that one edge of the substrate contacts with the thin film.
FIG. 19 is a cross-sectional view of a conventional solid-state image pickup device disclosed in Japanese Laid-Open Patent Publication No. 2005-316111.
Japanese Laid-Open Patent Publication No. 2005-316111 discloses that a transparent resin film is formed so as to entirely embed a plurality of microlenses, as shown in FIG. 19, by using the spin coating method.